The high intensity sodium vapor lamp with which the invention is most useful comprises a slender tubular ceramic arc tube which is generally mounted in an outer vitreous envelope or glass jacket. The arc tube is made of light-transmissive refractory oxide material resistant to sodium at high temperatures, suitably high density polycrystalline alumina or synthetic sapphire. The filling comprises sodium together with mercury for improved efficiency, and a rare gas to facilitate starting. The ends of the tube are sealed by closure members through which connections are made to thermionic electrodes which may comprise a tungsten coil activated by electron-emissive material. The outer envelope which encloses the ceramic arc tube is generally provided at one end with a screw base to which the electrodes of the arc tube are connected.
The high pressure sodium vapor lamp contains an excess amount of sodium mercury amalgam, that is it contains an excess amount of sodium mercury amalgam, that is it contains more amalgam than is vaporized when the lamp reaches a stable operating condition. By having an excess, the vapor pressure is determined by the lowest operating temperature in the arc tube and the quantity supplied is not critical. In long lived lamps it is customary to provide sufficient amalgam so that shifts in the sodium-mercury ratio with life due to any burnup or loss of sodium from the arc tube are negligible. In lamp manufacture, generally from 15 to 30 milligrams of amalgam with a sodium content between 10 and 30% by weight are provided.
The excess amalgam in the arc tube condenses at the cold spot whose location is determined by the heat balance in the lamp. If the lamp's orientation places the cold spot lowermost, gravity helps to retain the condensate in one place. However in universal burning lamps which are sold for operation in any orientation or attitude, the cold spot is frequently now lowermost, and with the quantity of amalgam usually provided, such lamps are sensitive to shock and vibration. Shock can cause all or part of the excess amalgam to be temporarily displaced to a hotter location, producing a sudden increase in the operating pressure of sodium and mercury which entrails a rise in voltage drop across the arc tube. The rise can be severe enough to exceed the sustaining voltage of the ballast, in which case the lamp extinguishes. When the lamp goes out in this way, commonly called drop-out, it cannot be restarted until it has cooled and that may take from 1 up to 10 minutes, depending on the ambient temperature. In extreme cases where vibration causes a droplet of amalgam from the cold spot to be projected into the very hot region forward of the electrode tip, the thermal shock on the hot ceramic may be enough to crack it. Lamp applications which are particularly difficult with regard to shock and vibration include highway bridges, loading docks, heavy machinery, and railyard lighting.
Some lamps utilize a projecting sealed-off metal exhaust type external to the arc tube proper as a reservoir for excess sodium mercury amalgam. In such lamps, sensitivity to vibration may be reduced by crimping the external exhaust tube at an intermediate point. The crimp leaves only restricted channels communicating with the reservoir portion, allowing the passage of amalgam in vapor form but preventing its movement as a liquid, as taught in U.S. Pat. No. 4,065,691--McVey, 1977. However, other ceramic lamp constructions which do not have an external metal exhaust tube cannot use that feature to reduce sensitivity to vibration. In lamps utilizing at both ends a wire seal such as taught in U.S. Pat. No. 4,034,252--McVey, 1977 and having no external amalgam reservoir, excess amalgam collects within the arc tube proper, generally at one end which the heat balance makes the cold spot. If the lamp is operated in an orientation putting such end uppermost, it may be quite sensitive to shock and vibration.